Method of anodizing a thin-film device

ABSTRACT

Thin-film resistors and other devices are precision adjusted by a regular, geometric progression of steps. Each step of a binary step anodization pattern for adjusting a resistor to a nominal resistance value is designed to decrease by one-half the percentage deviation in resistance from the nominal value at the end of the preceding step. Successive binary step decreases in the percentage resistance deviation are attained by a series of binary step decreases in the product of anodizing current and anodizing time for each step. The resultant, decreasing rate adjustment pattern tends to render negligible any chance of substantially overshooting the desired nominal value while permitting rapid attainment of the nominal value. A general purpose, process control computer is equipped with a series of tapes or other program devices storing anodizing current and anodizing time information for carrying out successive steps of the subject anodizing technique. Such information is that which has been derived for given sets of conditions and resistor codes, incorporating various regular, geometric progression anodizing patterns and linear approximations, such as a linear approximation of a binary step pattern. Each resistor is anodized in successive steps, after each of which steps a resistance measurement is made and the anodizing current and anodizing time for the next step are determined. Anodization is terminated when the tolerance zone about the nominal resistance value is entered.

Unite ttes Manning et al.

tent

[ 1 Jan, 18, W72

[22] Filed:

[54] METHOD OF ANODHZHNG A THIN-FILM DEVICE [72] Inventors: Robert A. Manning, Essex, Mass; Donald H. Raymond, Rockingham, NH.

[73] Assignee: Western Electric Companyflncorporated,

New York, NY.

Nov. 6, 1970 [2]] Appl. No.: 87,370

Primary Examiner-John H. Mack Assistant Examiner-R. L. Andrews Attorney-W. M. Kain and R. P. Miller [57] ABSTRACT Thin-film resistors and other devices are precision adjusted by a regular, geometric progression of steps. Each step of a binary step anodization pattern for adjusting a resistor to a nominal resistance value is designed to decrease by one-half the percentage deviation in resistance from the nominal value at the end of the preceding step. Successive binary step decreases in the percentage resistance deviation are attained by a series of binary step decreases in the product of anodizing current and anodizing time for each step. The resultant, decreasing rate adjustment pattern tends to render negligible any chance of substantially overshooting the desired nominal value while permitting rapid attainment of the nominal value.

A general purpose, process control computer is equipped with a series of tapes or other program devices storing anodizing current and anodizing time information for carrying out successive steps of the subject anodizing technique. Such information is that which has been derived for given sets of conditions and resistor codes, incorporating various regular, geometric progression anodizing patterns and linear approximations, such as a linear approximation of a binary step pattern. Each resistor is anodized in successive steps, after each of which steps a resistance measurement is made and the anodizing current and anodizing time for the next step are determined. Anodization is terminated when the tolerance zone about the nominal resistance value is entered.

15 Claims, 5 Drawing Figures PATENIEDJAMIBYZ 3.635802 SHEET [1F 4 MEMORY CALCULATE RESISTANCE RATIOS R FOR T1+-n 1' RECORD R1 FOR MATCHING FOR MATCHING R FOR NEXT RESISTOR MEMORY: ABSOLUTE VALUES OF R RECORD IIHIGHHI RECORD "REJECT" AND D RECORD RP HALT AND 70D PATH EMPLOYED TO SORT RESISTORSWITH- OUT ANODIZATION COMPUTE I AND T Lab YES H RECORD H STALLED ANOD! ZE 1 METHOD OF ANODIZING A THIN-FILM DEVICE BACKGROUND OF THE INVENTION This invention relates to methods of anodizing a thin-film device and, more particularly, to methods of anodizing a thinfilm device according to a selected pattern of anodizing steps to adjust a parameter of the thin-film device to a desired value.

In the manufacture of thin-film devices, such as thin-film resistors, a high degree of precision is often required either in the absolute value of a parameter of a finished device or in matching values of the parameter in two or more finished devices. In the case of certain high-precision thin-film resistors for example, tolerance requirements for finished resistors may be of the order of i0.0l percent of a design resistance value or of the resistance value of a resistor to be matched. A wellknown method of adjusting thin-film resistors to value involves anodization, a process according to which a portion of the thickness of a resistor, which may be made of a thin film of tantalum, tantalum nitride, or other material, is oxided to form a dielectric layer, thereby reducing the thickness of the remaining, unoxidized portion of the film to increase the resistance of the film.

A typical method of manufacturing thin-film resistors, generally utilized in the production of resistors with less demanding tolerance requirements, in the area of $0.5 percent or more, employs a two-stage resistance adjusting operation. A first stage of resistance adjustment involves the anodization of each thin-film resistor at a first, relatively high rate by a series of relatively high current anodizing steps which may be interspersed with resistance measuring steps. This first stage of anodization continues until the resistance of the thin-film resistor is found by measurement to have been increased to within a first rough tolerance range of the nominal resistance value, typically -5 percent. A second stage of resistance adjustment then provides a more precise, relatively low rate anodization by a series of relatively low current anodizing steps. This second stage of anodization continues until the resistance of the thinfilm resistor is found by measurement to fall within the typical :05 percent or more tolerance requirement. In order to provide a more efficient usage of testing and anodizing equipment so as to increase the number of thin-film resistors produced in any given time period by this process, a multiplexed system is often utilized, whereby a number of thin-film resistors are anodized simultaneously with sequential testing determining instantaneous resistances of the resistors during anodization.

It should be clear that, even with the utilization of multiplexing techniques, overly long resistor processing times would be characteristic of the procedure outlined above, if such procedure were to be attempted in the manufacture of resistors having considerably more exacting tolerance requirements, e.g., iflOl percent of nominal resistance value. The second stage of anodization would require an extremely low rate of resistance change to avoid overshooting the precise tolerance zone specified. A more efficient method of highprecision anodization for achieving desired values for parameters of thin-film devices, e.g., resistances, in a relatively short period of time is highly desirable. Such method must, of course, also provide good repeatability characteristics and be as simple as possible in nature for utilization in a large scale program of anodization.

SUMMARY OF THE INVENTION An object of the invention resides in new and improved methods of anodizing a thin-film device, e.g., in order to adjust a parameter of the device, such as resistance, to a desired value.

The invention contemplates the anodization of a thin-film device in accordance with a selected pattern of anodizing steps which will bring a parameter of the device, e.g., the resistance of a thin-film resistor, within a very narrow tolerance zone in a minimum time period, yet with a minimal likelihood of overshooting the nominal value of such a degree as to pass out of the tolerance zone. The contemplated pattern of anodizing to accomplish these ends involves a multiple step procedure under which each successive step is designed to decrease by a fixed fractional multiple the percentage deviation of the parameter of interest from a desired final value of the parameter. Such pattern is achieved by a series of steps wherein the product of anodizing current and anodizing time for each succeeding step is equal to such fixed fractional multiple of the product of anodizing current and anodizing time for each preceding step. A preferred fixed fractional multiple utilized in carrying out the method of the invention is equal to one-half, such that the percentage deviation of the parameter of interest from the desired value is decreased continually in binary fashion. The binary pattern has been selected for its capability of changing rapidly the value of the parameter of interest, while eliminating any chance of shooting the desired nominal value by more than one binary step, an insignificantly small quantity once adjustment has brought the parameter into the vicinity of the desired nominal value.

The invention further contemplates adjustment of thin-film devices according to a pattern of steps of decreasing the anodizing current and anodizing time in binary fashion or other geometric progression wherein one of the anodizing current and the anodizing time may be held constant during several successive steps while the other is decreased in accordance with the pattern. For example, a method in accordance with the principles of the invention employs a number of steps wherein constant anodizing current is applied to a thin-film device for time periods decreasing in fixed geometric progression, followed by a number of steps wherein the anodizing time period is maintained constant and the anodizing current is decreased according to the fixed geometric progression. Anodization may be interrupted briefly after each anodization cycle during this process, e.g., after each step, to allow testing equipment to check the progress of the thin-film device toward nominal value. Multiplexing may be utilized in connection with such method to produce finished anodized thin-film devices at an increased rate.

BRIEF DESCRIPTION OF THE DRAWING FIG. I of the drawing is a plot depicting the first three steps of a pattern of anodization illustrative of the principles of the invention, the plot displaying an anodizing current-time product versus the percentage deviation from a nominal resistance value of a thin-film resistor;

FIG. 2 constitutes an expanded vertical scale continuation of the plot of FIG. I, depicting several additional steps according to the FIG. I pattern of anodization;

FIG. 3 is a plot of anodizing time versus percentage deviation from value for the first three steps of the pattern of FIGS. 1 and 2;

FIG. 4 is a plot of anodizing current versus percentage deviation from value for several additional steps of the pattern DETAILED DESCRIPTION Turning now to FIGS. 1 and 2, it is desired, firstly, that a thin-film device, such as a thin-film resistor, be anodized to adjust a characteristic parameter of the thin-film device, such as its resistance, to within a very close tolerance, say 10.01 percent, of a desired, nominal value or of a value in fixed ratio to the value of the parameter in a thin-film device to be matched. The specified tolerance zone is represented at the right-hand end 21 (FIG. 2) of the horizontal scale used in the plot of FIGS. I and 2, FIG. 2 constituting an expanded vertical scale, rightward continuation of the portion of the plot shown in FIG. 1. A potential range of percentage deviations from a desired value of a parameter, beginning at =5 percent and ending at about =0.0l percent, is depicted along the horizontal scale of FIGS. 1 and 2.

It is desired, further, that the parameter of the thin-film device be adjusted to within the very close tolerance of the nominal resistance value in as short as possible a total time period, yet with a minimal likelihood of overshooting the tolerance zone associated with the nominal value.

The methods hereinafter described for carrying out the principles of the invention will be discussed in terms of anodizing to adjust the resistance of a thin-film resistor to within 10.01 percent of a desired, nominal resistance value, starting with an initial deviation from value in the neighborhood of 5 percent. It should be understood, however, that such methods might also be employed in the adjustment of parameters, such as capacity, of other thin-film devices, such as thin-film capacitors, and that the selected percentage deviation values of il percent and percent are merely representative of a wide range of possible end points, which might be employed in performing these methods.

The pattern or anodizing steps depicted in FIGS. 1 and 2 has been devised to accomplish the desired end of quickly, yet reliably, producing 100] percent tolerance thin'film resistors, starting with a 5 percent resistance deviation. Ten successive process steps are each represented by a different step number "=0 through n=9. Each step n is characterized by a different horizontal extent. Thus, step n=l continues from a 5 percent deviation in resistance value to 21 -25 percent deviation, while step n=2 extends from the 2.5 percent deviation to a l .25 deviation and a step n=3 goes from the -l .25 percent deviation to a 0.625 percent deviation. Each successive step may be seen to reduce by one-halfthe percentage deviation in resistance existent at the end of the preceding step.

It should be clear from FIGS. 1 and 2 that a series of i0 anodizing steps, which may be performed in rapid succession according to a binary step pattern of anodization, will reduce the percentage deviation from 5 percent to within $0.01 percent of the nominal value of a thin-film resistor. Owing to the extremely small nature of each binary step increment as the desired value, is approached, any chance of overshooting the tolerance range associated with such desired value, even under the nonideal conditions involved in large scale production or resistors, is minimal. Thus, any errors in resistor measurements, as well as any inexactness in the application of the theoretical process to actual production practice with respect to any particular thin-film resistor undergoing anodization, should affect the final resistance value of the resistor by no more than one binary step, i.e., at most something of the order of 0.01 percent of the nominal value for a 0.02 percent tolerance zone. Such effect would still fall within the tolerance zone for the resistor.

Represented by the vertical scale of FIGS. 1 and 2 is a product I,,T,, of anodizing current I and anodizing time T for each step n. In order that each successive step will reduce the percentage deviation in resistance from nominal value by onehalf, it has been determined that the product of anodizing current and anodizing time should be reduced in binary fashion with each succeeding step. Thus, as may be seen in FIGS. 1 and 2, the product I,T, of anodizing current and anodizing time for step n=l is selected to be one-half of the product I T of anodizing current and anodizing time for step 11 0, while the analogous product T for step n=2 is one-half of the product I,T,. The product I,,T,, of anodizing current and anodizing time may be seen to follow the general formula where l and T are, respectively, the anodizing current applied to the thin-film resistor during an initial step corresponding to n=0, and the time for which such anodizing current is applied during the initial step. This pattern of binary decreases in the current-time product for successive anodizing steps has been selected in order that the desired binary pattern of decreases in percentage deviation from nominal resistance, provides rapid anodization such that a minimal likelihood of overshooting the tolerance zone, will result.

In order to satisfy the formula I,,T,,=2l 7' it should be clear that the pattern of variation in the product I,,7',, with succeeding steps may be achieved by varying one or both of the variables from step to step. One example of a method of anodization according to such pattern is illustrated in FIGS. 3 and 4 ofthe drawing. A first phase of anodization in accordance with this method (FIG. 3), encompassing steps n=0 through n=3 is characterized by a binary step pattern of decreasing anodizing time T while the anodizing current 1,, for each step is held constant at a preselected initial value l A second phase of anodization in accordance with the method (FIG. 4), involving steps n=4 through n=9, is characterized by a binary step pattern of decreasing anodizing current I,,, wherein the anodizing time period for each step is held constant at the value T lfl utilized during the last step, n=3, of the first phase of anodization, and wherein the anodizing current for the first step, n=4, of the second phase is set at 1 /2. Thus, the overall anodizing pattern provided by FIGS. 3 and 4 is in accordance with the overall formula l,,T,,=2 "l,,T

An example involving the selection of various numerical values in order to carry out an anodizing pattern generally of the type of FIGS. 3 and 4 will now be described. An approach is employed in which a substantially final anodizing step, say n=N, is first postulated. Then, a series of steps, n=N-l, n=N- 2, etc., are derived by working backwards until values for anodizing current I and T have been derived for use in a first step, n=0, which will correspond to a step rr-NN. Thus, using a 0.02 percent resistance deviation (D) as a starting point, it is desired to anodize to a value D==0.0l percent dur ing a final or next to final process step n=N, whereupon the tolerance zone about the nominal resistance value will have been entered. The selection of step n=N is intended to assure that final precision anodization at D= -().0l percent will not result in any overshoot of a nominal resistance value by more than one binary step. A typical thin-film resistor material might have a resistivity of 50 ohms per square, a resistivity which has been found empirically to require a voltage rise of 0.0167 volts to make a 0.01 percent change in resistance deviation at the point D=0.02 percent. A typical minimum anodizing period to be utilized is 25 milliseconds, a period still providing a feasible anodizing time for accurately timed, anodization switching operations to take place. A rate of voltage rise is calculated by dividing the 0.0l67 volts voltage rise by the 25 millisecond minimum time period value, the calculated rate of voltage rise being 0.667 volts per second, or 40 volts per minute. These initial conditions are set forth as step n=N in the table shown below, in which the percentage resistance deviation (D) is doubled with each succeeding step in the reversed step order, N, Nl, N2, etc., and in which the rate of voltage rise in volts per minute (v./min.), as may be seen in the table, is permitted to double with each succeeding step, for steps Nl N2, N3, N4, NS, with the anodizing time '1,, maintained at its minimum value of 25 milliseconds to provide a series of rapid anodizing steps.

The doubling of several reversed-order steps N+l through N-5, as shown in the table, does not continue for steps N6, N7, and N8. An absolute maximum rate of voltage rise permissible is set at 2,000 volts per minute, since any voltage rise at an appreciably greater rate results in damage to oxide film due to a voltage breakdown and subsequent crystallization of the oxide layer. It is clear that step N6 would be characterized by a rate of voltage rise of 2,560 volts per minute, a value in excess of the 2,000 volts per minute absolute maximum, if the constant anodizing time phase (Phase II) of the method were continued in the reversed order progression of steps. Thus, step N-5 is takenas a crossover point X, at which the constant anodizing time phase (Phase II) will begin in the forward order actual performance of the calculated pattern. Step N 6 is assigned to Phase I of anodization, the constant anodizing current phase. Calculation of the values of anodizing time T, for steps N6, N7, etc., follows by doubling T, in each reversed order step, v./min. and 1,, remaining constant, until a percentage resistance deviation of 5.l2 percent, greater in absolute value than 5 percent is attained for step n-8 whereupon the calculations may cease. Step N8 is now seen to correspond to step n=0 and the 200 millisecond value of T,, at step N8 is taken to be T An initial anodizing current of a relatively high value I, may 7 now be obtained from the empirically derived relationship I=A V/S ,000-

where A is the area in square mils of the thin-film resistor to be anodized, V is the rate of voltage rise in vol'ts per minute and I is current in microamps. Thus, for a value of V equal to 1,280, as in the four steps N.5 through N8 of Phase I of the method, 1, corresponding to the relatively high current value 1 is equal to 0.256A.

The calculations and tabulation provided in the above example for thin-film resistor material of 50 ohms per square resistivity may be duplicated to cover a wide range of materials of differing resistivity values. By way of example, another table might be prepared for materials of ohms per square resistivity. Such table would exhibit a crossover point X at step n=4, where step n==l corresponds to an initial percentage deviation in resistance D of about 5 percent, e.g., 5.l2 percent.

It should be noted that the anodizing patterns depicted in I FIGS. 3 and 4 have been set up for illustrative purposes only and do not represent the plotted results of any actual tabulation of the type just described. In actual practice, an assumed initial value of 5.l2 percent for deviation D appears to be preferable to the 5 percent value of FIG. 3 (and FIG. 1) simply due to the fact that 5.l2 is an exact binary multiple of 0.01, i.e., 0.0lX2.

An advantageous manner of employing a method in accordance with the principles of the invention involves the use of a conventional process control computer having a memory, or peripheral systems such as an associated library of tapes, storing information derived in accordance with the above calculations and tabulation for a wide range of materials of differing resistivity values. Initial values of anodizing current I and anodizing time T plus proper crossover points x, are provided for the various resistivities. Intermittent resistance tests may be interspersed between anodizing steps through the use of any conventional measuring circuitry. Exemplary of such circuitry are either intermittent, anodize and then test circuits,

such as the circuits disclosed in US. Pat. Nos. 3,341,444 anclv 3,341,445, both issued on Sept. 12, 1967, respectively in the names of E. A. LaChapelle and AR. Gerhardt and both assigned to the assignee of the instant invention, or circuits permitting concurrent anodization and resistance testing. The resistance tests will indicate the progression of anodization along the horizontal scale, percentage resistance deviation, of FIGS. 3 and 4, aiding in controlling the anodization to the particular control pattern in use for the resistor.

Multiplexing may be advantageously employed to anodize a number of thin-film resistors simultaneously with a momentary coupling of each successive resistor undergoing anodization into testing relationship with the resistance measuring circuitry. The particular apparatus to be used is not material to the methods of the invention.

Shown in FIGS. 3 and 4 are a pair of dotted lines a and b which constitute difierent continuous, linear approximations of the nonlinear formula I,,T,,=2-"I,T, characterizing the solid line, binary step pattern depicted in FIGS. 3 and 4. Any such, linear approximation may be ad- I vantageously employed in practicing the methods of the in-,

and FIG. 4 horizontal scale, percentage resistance deviation,

specifying l,, and T, conditions to be employed during the next anodizing step according to the substantially binary step anodizing pattern of the dotted line b.

It should be understood that the general principles of the invention relate to anodization according to nonbinary, as well as binary, geometrical progression patterns, providing a continually decreasing product of anodizing current and anodizing time in successive step. A general formula for anodization in accordance with the invention would be 1,, T,,=K"l T wherein K is a constant greater than one and wherein all the other terms have already been defined. The formula I,,T,,=2 "I T defining the binary step approach is, thus, seen to be merely a specialized case of the general formula I,,T,,=k "I T wherein K is taken as the integer 2. Other integer or noninteger values of K, greater or less than 2, might be employed in regular geometric progression patterns of anodization in accordance with the principles of the invention to cover those situations which involve requirements relating to lesser or greater degrees of accuracy, of risk of overshooting tolerance zones, or of rapidity of anodization than those provided by the binary step pattern of FIGS. 1-4.

Turning now to FIG. 5 of the drawing, a typical application of the subject matter of the invention in manufacturing a number of 10.01 percent tolerance tantalum nitride thin-film resistors will next be described. FIG. 5 constitutes a flow chart illustrating a series of operations, which may be performed by a conventional, general purpose, process control computer device, in the production of such resistors. The operations to be performed according to this flow chart are to anodize precisely resistors previously roughly adjusted to approximately a -5 percent tolerance, utilizing conventional anodizing circuitry and electrolytes, e.g., an electrolyte composed of 0.01 percent citric acid solution, in a carboxyl cellulose vehicle.

Certain terms, not previously defined, are utilized in the flow chart of FIG. 5. The term Rp represents the programmed nominal resistance value for any individual thin-film resistor,

. n having a resistance R,. Numerical values of the subscript l will be used to identify specific resistors, for example, resistors r and r having resistances R and R respectively. The term i refers to the total number of resistors to be anodized in a particular group of resistors. The tolerance zone about the nominal value Rp for each resistor n, i.e., the 10.0I percent deviation zone 0.9999 Rp R, I.000l Rp, is defined as the limit range L. The term n indicates a maximum number of anodizing steps to be permitted with respect to any resistor r,.

The anodization of a number of i of thin-film resistors commences at START" position 31 in FIG. 5. The memory of the computer apparatus, for example, asprovided' by a program tape associated with a particular code of resistor to be anodized, is searched (box 32) to provide (box 33) a programmed nominal resistance value Rp for the first resistor r Next (box 34), the resistance R, of the resistor r is measured by conventional resistance testing equipment. Such equipment determines whether the resistance R, is more than a value of 5 percent above the resistance Rp programmed for the resistor r, (box 36); if not, whether the resistance R is greater than the maximum resistance of the programmed tolerance zone L associated with the resistor r i.e., whether R l.000l Rp for the resistance Rp programmed for the re 'sistor r (box 37); and, if not, whether the resistance R initially falls-within the programmed tolerance zone L associated with the resistor r i.e., whether 0.9999 Rp R, l.000l Rp for the resistance r, programmed for the resistor r (box 38).

If the conventional resistance testing equipment indicates (box 36) that the resistance R, is initially excessive by over 5 percent, the computer is programmed to indicate that the resistor r, is HIGH" (box 41) by activating conventional recording mechanisms (not shown). If the resistance R, is

determined (at box 37) to be initially above the maximum resistance of the tolerance zone L, a similar REJECT" recordation (box 42) occurs. If the resistance R, is measured as falling within the tolerance zone L about the programmed resistance Rp for the resistor r,, (at box 38) the values of Rp and percentage resistance deviation (percent D) are recorded (box 43). In any of these three cases, the computer apparatus will determine, after recordation, whether the individual resistor r, just examined, in this instance r,, is the last scheduled to undergo examination, i.e., whether i i,,,,,,, (box 44), preparatory to addressing a next resistor.

Thus far, a program has been described for sorting resistors r, by selecting an appropriate one of several different categories. It may be considered advantageous in certain situations to run a large number of resistors through this sorting program in rapid succession without any anodization occurring. Those resistors not meeting the test conditions of boxes 36, 37 and 38, Le, resistors for which the resistance R, is initially less than that at the lower end of the tolerance zone L, may later be anodized to value. In such circumstance, the dotted line path 46 depicted in FIG. 5 would be employed as the path from box 38 where the condition of box 38, R,- sl L1, is not met.

It may be noted that the determinations of boxes 36 and 37 are not necessary for an anodization program, but are useful only in a sorting operation, which may be performed either separate from or as a part of a program of anodization.

Returning now to the anodization of resistors, e.g., after the carrying out of a sorting program and eliminating all resistors for which the initial resistance r, is equal to or above the resistance of the lower end of the tolerance zone programmed for the resistor, it may be assumed that the test R,- sl L1 of ox 3?. t l fi saiiN Ql i q q tienllliqsq lauts s thereupon directed by its program to compute (box 47) values of I,, and T which will be utilized in a first anodizing step (as directed by box 48).

The program tape associated with the particular code of thin-film resistor to undergo anodization provides the computer with the information of the type illustrated by the plots of FIGS. 3 and 4 and the calculations and tabulation previously described. It is assumed that linear approximation b" of the nonlinear binary step pattern has been selected as a most convenient pattern of anodization to be employed for this code of resistor. The conventional resistance testing equipment indicates a percentage deviation in resistance R, of the resistor r, from the resistance Rp programmed for the resistor r, this percentage deviation corresponding to a first position along the horizontal scale, presumably in FIG. 3. Let us say that the corresponding point on dotted line b is that shown in FIG. 3 at 51. The anodizing time T,, determined by the computer for use in a first anodizing step corresponds to the position of the point 51 with respect to the vertical scale of FIG. 2. The initial anodizing current 1,, is the value 1 as determined from the formula I=Ai /',000 in the manner discussed previously, programmed into the tape.

Once I,, and T values have been selected for a first anodizing step according to the linearized, generally binary pattern b of FIGS; 3 and'4, the program directs the computer to carry out a first anodizing step (box 48), the resistor being anodized in conventional manner in the presence of an electrolyte after which a new resistance value R, of the resistor r, is measured (box 34). If anodization has resulted in the predicted binary pattern of decrease in percentage deviation in resistance, point 52 on dotted line b will have been reached.

The new value of R, is now subjected to the comparisons of boxes 36, 37, and 38. Since the conditions of these boxes are not met, the steps required by boxes 47 and 48 must be repeated. Calculating, anodizing, measuring and comparing operations will next continue, presumably through approximately the predicted anodization step end points 53, 54, 55, 56, 57, etc., until the comparison called for by box 38 indicates that the resistor r, has had its resistance R, increased to fall within the tolerance zone L about the programmed nominal value Rp for the resistor r,. A changeover from a constant anodizing current first phase of anodization to a constant anodizing time second phase of anodization at an appropriate time directed by the program according to dotted line b, i.e., at point 54. The constant anodizing time will have thereafter been the predetermined value of 25 milliseconds.

It should be observed that the attainment of the predicted intermediate points 52, 53, 54, etc., is far from critical. The binary step approach has been especially selected for its selfcorrective nature. Each succeeding error, if any errors are present, will affect the anodization by only a fraction of a binary step. The last of a series of anodizing steps should, thus, still bring the resistance of the resistor r, within the tolerance zone L, that is 0.9999Rp R, 1.0001Rp.

A control over any situation in which the lowest resistance of the resistance tolerance range L is not attained in an excessive number of steps n is provided by the subject matter of boxes 61 and 62 of FIG. 5. Boxes 61 and 62 involve the making of a comparison of the number of anodizing steps n, after each step, with the number n and the signalling of associated recording equipment to indicate a "stalled condition once the number n of steps has been equalled.

After the resistance r, has been trimmed to value, a comparison of the resistor number i=1 is made with the number of resistors to be anodized. Since more than one resistor is to be anodized, the resistor r, is found not to be the last resistor. Thereupon, the computer program for the particular code of resistor is consulted (box 63) to determine whether the final resistance of the first resistor r, is to be matched by the next resistor r,, or by any other succeeding resistor. If not, boxes 32 and 33 are again addressed to provide a programmed nominal resistance value Rp for the next resistor r to be anodized, whereupon the entire process is repeated with respect to the resistor r and then to any succeeding resistors. If so, the value of R, is recorded (box 64). Matching operations will thereafter occur through the use, in anodizing the matching resistor, of a value of a target nominal resistance value Rp corresponding to the recorded value of R,, multiplied (box 66) by a desired resistance ratio, if other than unity, provided by the computer memory (box 67), e.g., in the form of the program tape for the code of resistor undergoing anodization. Such matching operation might be employed in the manufacture of thin-film resistor networks including the resistors r,, r,, etc.

Anodization of successive resistors r,, r etc., continues until a comparison at box 44 indicates that a final resistor has been trimmed to value. Thereupon, the anodizing operation is terminated (box 68).

While the various resistors have been anodized serially according to the example described above, it should be clear that the described example is easily adaptable to a multiplexed arrangement wherein simultaneous anodization of two or more resistors would occur. Even without multiplexing, however, an extremely rapid, yet reliable, pattern of anodization may be obtained from the practice of the method of the invention in the mannerjust described.

It is to be understood that the methods described above are simply illustrative of certain embodiments of the invention. Many modifications may be made without departing from the invention.

What is claimed is:

1. A method of anodizing a thin-film device to adjust a parameter of the device to within a tolerance range of a desired final value, which comprises:

performing a plurality of steps each comprising applying to the thin-film device in the presence of an electrolyte an anodizing current for an anodizing time, the particular claim 1, wherein said fixed fractional multiple is one-half.

3. A method of anodizing a thin-film device as set forth in claim I, wherein the product of anodizing current and anodizing time for each succeeding step is selected as said fixed fractional multiple times the product of anodizing current and anodizing time for the preceding step.

4. A method of anodizing a thin-film device as set forth in claim 3, wherein said fixed fractional multiple is one-half.

5. In a method of anodizing a thin-film device, the steps of:

applying to the thin-film device in the presence of an electrolyte a first anodizing current for a first time period selected to provide a first current-time product; and then applyirig to the thin-film device in the presence of said electrolyte a second anodizing current for a second time period, the second anodizing current and the second time period selected to provide a second current-time product substantially equal to one-half of said first current-time product.

6. In a method of anodizing a thin-film device:

anodizing the device by performing at least three steps substantially according to a pattern of steps where n is the number of each step, 1,, is the anodizing current applied to the device in the presence of an electrolyte during each step n, T, is the time for which the anodizing current 1,,is applied during each step n, K is a constant greaterthan I, I is the anodizing current applied to the device during an initial step corresponding to n=(), and T is the time for which the anodizing current I is applied during said initial step.

7. In a method of anodizing a thin-film device as set forth in claim 6, where K=2.

8. In a method of anodizing a thin-film device:

anodizing the device by performing at least three steps according to a linear approximation of the nonlinear pattern of steps 9. In a method of anodizing a thin-film device as set forth in claim 8, where K=2.

10. In a method of anodizing a thin-film device, the steps of:

applying to the thin-film device in the presence of an electrolyte a first anodizing current for a first time period, the first anodizing current and the first time period selected to provide a first current-time product; then applying to the thin-film device in the presence of said electrolyte a second anodizing current for a second time period, the second anodizing current and the second time period selected to provide a second current-time product substantially equal to a predetermined percentage of the first current-time product; and then applying to the thin-film device in the presence of said electrolyte for a third time period, the third anodizing current and the third time period selected to provide a third current-time product substantially equal to said predetermined percentage of the second current-time product.

11. In a method of anodizing a thin-film device, as set forth in claim 10, said predetermined percenta e being 50 percent.

12. In a method of anodizing a thin-fi m device as set forth in claim 10, at least one of said steps comprising varying the anodizing current from that employed during a preceding step.

13. In a method of anodizing a thin-film device as set forth in claim 10, at least one of said steps comprising varying the time period from that employed during a preceding step.

14. Ina method of anodizing a thin-film device, as set forth in claim 10, to bring a parameter of the thin-film circuit within a tolerance range of a desired final value of the parameter:

performing a plurality of steps each comprising applying to the thin-film device in the presence of said electrolyte an anodizing current for a time period such that the currenttime product for each succeeding step is substantially said predetermined percentage of the current-time product for the preceding step until the parameter falls within the tolerance range of the desired final value of the parameter, at least one of said steps comprising varying the anodizing current from that employed during a preceding step and at least one of said steps comprising varying the time period from that employed during a preceding step.

15. In a method of anodizing a thin-film device as set forth in claim 10:

first performing a plurality of steps each comprising applying to the thin-film device in the presence of said electrolyte a constant anodizing current for time periods varying substantially in geometric progression according to said predetermined percentage; and then performing a plurality of steps each comprising applying to the thin-film device in the presence of said electrolyte for a constant time period anodizing currents varying substantially in geometric progression according to said predetermined percentage.

D T sTATTs PATENT eTTTeR Patent No, 3,635p8o2 Dated January 18, 1972 memo) ROBERT A. MANNING and DONALD Ht RAYMOND it is certified that error appears in the above idemified patent and that said Letters Patent are hereby ecrrected as shown below:

Column 2, line 75 cancel =5" and insert -5 Column 3, line 1 cancel "=0.0l" and insert -0.0l any Column 4, lines 52 and 60, cancel "v,/min. and insert "V/minn". Column 5, line 13 cancel "v.,/min. and insert --V/min.--. Column 6, line M after "carboxyl" insert --metncl and lines 55 and 7h, after "RE" (first occurrence) and after "R- ",insert 5. Colum 7,

line 15, after "i' (first occurrence) insert 2 "5 and Column 7, lines 27 and 37 cancel 'R Q -l 'L 1" and insert "R L Column 8, line 21, after O.9999R c" insert 5 and after "R ins rt s and line 28, cancel "stalled" and insert -"'ST ALLED Column 9, line 5 cancel "i and insert "I Signed and sealed this 5th day of February 1974.

(SEAL) Attest:

EDWARD M.FLETCHE-R,JR. RENE D. TEGTMEYER Attestlng Officer Acting Commissioner of Patents 

2. A method of anodizing a thin-film device as set forth in claim 1, wherein said fixed fractional multiple is one-half.
 3. A method of anodizing a thin-film device as set forth in claim 1, wherein the product of anodizing current and anodizing time for each succeeding step is selected as said fixed fractional multiple times the product of anodizing current and anodizing time for the preceding step.
 4. A method of anodizing a thin-film device as set forth in claim 3, wherein said fixed fractional multiple is one-half.
 5. In a method of anodizing a thin-film device, the steps of: applying to the thin-film device in the presence of an electrolyte a first anodizing current for A first time period selected to provide a first current-time product; and then applying to the thin-film device in the presence of said electrolyte a second anodizing current for a second time period, the second anodizing current and the second time period selected to provide a second current-time product substantially equal to one-half of said first current-time product.
 6. In a method of anodizing a thin-film device: anodizing the device by performing at least three steps substantially according to a pattern of steps InTn K nI0T0, where n is the number of each step, In is the anodizing current applied to the device in the presence of an electrolyte during each step n, Tn is the time for which the anodizing current In is applied during each step n, K is a constant greater than 1, I0 is the anodizing current applied to the device during an initial step corresponding to n 0, and T0 is the time for which the anodizing current I0 is applied during said initial step.
 7. In a method of anodizing a thin-film device as set forth in claim 6, where K
 2. 8. In a method of anodizing a thin-film device: anodizing the device by performing at least three steps according to a linear approximation of the nonlinear pattern of steps InTn K nI0T0, where n is the number of each step, In is the anodizing current applied to the device in the presence of an electrolyte during each step n, Tn is the time for which the anodizing current In is applied during each step n, K is a constant greater than 1, I0 is the anodizing current applied to the device during an initial step corresponding to n 0, and T0 is the time for which the anodizing current i0 is applied during said initial step.
 9. In a method of anodizing a thin-film device as set forth in claim 8, where K
 2. 10. In a method of anodizing a thin-film device, the steps of: applying to the thin-film device in the presence of an electrolyte a first anodizing current for a first time period, the first anodizing current and the first time period selected to provide a first current-time product; then applying to the thin-film device in the presence of said electrolyte a second anodizing current for a second time period, the second anodizing current and the second time period selected to provide a second current-time product substantially equal to a predetermined percentage of the first current-time product; and then applying to the thin-film device in the presence of said electrolyte for a third time period, the third anodizing current and the third time period selected to provide a third current-time product substantially equal to said predetermined percentage of the second current-time product.
 11. In a method of anodizing a thin-film device, as set forth in claim 10, said predetermined percentage being 50 percent.
 12. In a method of anodizing a thin-film device as set forth in claim 10, at least one of said steps comprising varying the anodizing current from that employed during a preceding step.
 13. In a method of anodizing a thin-film device as set forth in claim 10, at least one of said steps comprising varying the time period from that employed during a preceding step.
 14. In a method of anodizing a thin-film device, as set forth in claim 10, to bring a parameter of the thin-film circuit within a tolerance range of a desired final value of the parameter: performing a plurality of steps each comprising applying to the thin-film device in the presence of said electrolyte an anodizing current for a time period such that the current-time product for each succeeding step is substantially said predetermined percentage Of the current-time product for the preceding step until the parameter falls within the tolerance range of the desired final value of the parameter, at least one of said steps comprising varying the anodizing current from that employed during a preceding step and at least one of said steps comprising varying the time period from that employed during a preceding step.
 15. In a method of anodizing a thin-film device as set forth in claim 10: first performing a plurality of steps each comprising applying to the thin-film device in the presence of said electrolyte a constant anodizing current for time periods varying substantially in geometric progression according to said predetermined percentage; and then performing a plurality of steps each comprising applying to the thin-film device in the presence of said electrolyte for a constant time period anodizing currents varying substantially in geometric progression according to said predetermined percentage. 